Note: Descriptions are shown in the official language in which they were submitted.
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SECURE PACKET RADIO NETWORK
BACKGROUND OF THE INVENTION
THIS invention relates to a method of operating a network, typically a packet
radio network, which comprises a network operator station and a plurality of
user stations.
A network of this general kind is described in PCT patent application no. WO
96/19887 and comprises a plurality of stations which monitor each other's
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activity and transmit message data to one another, either directly or via
intermediate stations, in an opportunistic manner. One or more of the stations
can function as a network operator station which regulates the access of other
stations to the network and thus to desired destination stations.
In a commercial implementation of such a network it is necessary to uniquely
identify each station and to control its access to the network, both for
security
and billing purposes. This will prevent, for example, continued use of the
network by a subscriber whose account has fallen into arrears, and the
interception of messages by an unauthorised station.
The different stations may communicate via the same or different media. The
principle by which the stations generate their routing information is by
detecting other stations in their immediate vicinity, and monitoring the data
these stations send. By monitoring the contents of the data, a station will be
able to dynamically find routes to other stations in the network. This will
allow a station to send data to any other station, via any intermediate
station,
in the network even though it can not directly communicate with the
destination station.
If someone were to place an unauthorised station in the network, with an ID
that belonged to another station, it would cause routing problems and allow
the unauthorised station to intercept the data. Therefore it would be
necessary
to make sure that no unauthorised station could cause legitimate stations to
send it any data, and to ensure that a transmission from the unauthorised
station does not interfere with the dynamic routing tables in the legitimate
stations.
r i T
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SUMMARY OF THE INVENTION
According to the invention there is provided a method of operating a network
comprising a network operator station and plurality of user stations adapted
to
transmit message data to one another, directly or via intermediate user
stations, the method comprising:
generating at least one key required for use by the user stations;
transmitting to the network operator station, from a first user
station requiring a key, a key request message containing first
status data indicative that the message originates from a user
station lacking a key;
transmitting, from the network operator station to the first user
station, a key data message containing a key for use by the first
user station and second data corresponding to the first status data;
and
at any user station receiving the key data message, forwarding the
message to the first user station if the second status data thereof
meets at least one predetermined criterion.
The key request message from the first user station may be received by at
least
one intermediate station and forwarded to the network operator station if the
first status data thereof meets at least one predetermined criterion.
Preferably the key request message transmitted from the first user station
contains first status data identifying the key request message as a first
message
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transmitted by that station.
Similarly, the key data message transmitted by the network operator station
preferably contains second status data identifying the key data message as a
response to the key request message.
The method may include recording, at any user station receiving the key
request message, the identity of the first user station and the first status
data
thereof.
At user stations recording the first status data from the key request message,
data corresponding to the identity of the first user station is preferably
flagged
to indicate that the identity data may be used for no other purpose than the
transmitting of a key data message originating at the network operator station
to the first user station.
The key data message may comprise a network operator's public key which is
utilised by the first user station and all active user stations to decrypt
messages
from other stations which are encrypted with that key's corresponding private
key.
The key data message may further comprise a station public key and a station
private key allocated by the network operator to the first user station.
Messages transmitted from an originating station to a destination station are
preferably encrypted at least partially using at least one of the private key
of
the originating station, the public key of the originating station, and the
public
key of the destination station.
1
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Each user station may transmit a key probe signal from time to time, the key
probe signal containing identification data and the station public key of the
station transmitting the key probe signal, encrypted with the network
operator's private key, other stations receiving the key probe signal
decrypting
the signal using the network operator's public key to extract the
identification
data and station public key therefrom, for use when transmitting message data
to the station which transmitted the key probe signal.
The key request message may have a plurality of parameters which differ
from corresponding parameters of normal network messages. For example,
the message may have a different, preferably shorter, length than normal
messages and may have a different, preferably longer, time to die.
Further according to the invention there is provided a network comprising a
network operator station and plurality of user stations adapted to transmit
message data to one another, directly or via intermediate user stations, each
user station comprising a transceiver for sending data to and receiving data
from other stations in the network; and processor means for generating a key
request message for transmission to the network operator station, the key
request message containing first status data indicative that the message
originates from a user station lacking a key, and for receiving a key data
message from the network operator station containing a key for use by the
user station, thereby to enable the user station to communicate with other
stations in the network.
Each user station may include token reader means for reading identification
data from a secure token associated with a user, the identification data being
included in messages transmitted by the user station.
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The secure token may be a "smart card".
The invention extends to a user station adapted for use in a network
comprising a network operator station and a plurality of user stations adapted
to transmit message data to one another, directly or via intermediate user
stations, the user station comprising a transceiver for sending data to and
receiving data from other stations in the network; token reader means for
reading identification data from a secure token associated with a user; and
processor means for generating a key request message for transmission to the
network operator station, the key request message containing first status data
indicative that the message originates from a user station lacking a key, and
for receiving a key data message from the network operator station containing
a key for use by the user station, thereby to enable the user station to
communicate with other stations in the network.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a simplified block diagram of a transceiver unit which
functions as a user station in a network of the invention;
Figure 2 is a more detailed block diagram of the transceiver unit of
Figure 1;
Figure 3 is a simplified diagram illustrating the basic operation of the
network protocol; and
Figures are a more detailed flow chart illustrating the operation of the
4a to 4c
network protocol.
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,DESCRIPTION OF EMBODIMENTS
The present invention relates to a protocol for operating a network in which a
number of user stations transmit messages to one another, either directly or
via
intermediate stations. An example of such a network is described in PCT
patent application no. WO 96/19887.
Although the abovementioned patent application describes a packet radio
network it will be appreciated that the invention is applicable to other
networks in which user stations can communicate with one another via
intermediate stations in the network.
Networks of the kind referred to above can be utilised commercially, with
users being subscribers who are billed for their use of the network.
Alternatively, networks of this kind may be utilised by security forces such
as
police or military forces. These applications are given by way of example
only.
In almost all likely applications, the security of the network is important,
whether due to the need for a commercial operator to preserve the security of
clients' data and billing information, or due to the sensitive nature of
information transmitted in a military application, for example. It is also
important, in a commercial network, to maintain security for billing purposes,
so that only authenticated stations are able to utilise the network, and
permitting stations to be disabled in the case of non-payment of the user's
account, for example.
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To ensure security of data transmissions, each user station in the network
will
encrypt all packet headers (following the Sending ID), with its Private Key.
Every tenth probe (Key Probe) will not be encrypted, and will contain the ID
and Public Key of the user station that has been encrypted by the Network
Operator's Private Key (see below). Therefore any other user stations that
have the correct Network Operator's Public Key will be able to verify the ID
and Public Key of the user station.
All transmissions except for the occasional Key Probe will be encrypted. Key
Probes will not be used to adjust routing tables or any other adaptation
parameters. They will only be used to acquire the Public Key of the other user
stations.
User stations will not respond to Key Probes. They will only respond to
probes and packets that have been encrypted and verified.
The Network Operator's Public Key and the user station's own Public and
Private Keys must be acquired from the Network Operator when a user station
is first switched on. The Network Operator's Public Key will change on a
regular basis. Therefore a user station must always make sure that it has the
latest Network Operator's Public Key.
When a user station gets the Network Operator's Public Key from the
Network Operator, the Public Key will have a serial number, a renewal time,
an expire time, and a delete time. When the renewal time is reached the user
station must get the next Network Operator's Public Key. However it will
keep using the current Key until it expires. This will give all the user
stations a
chance to get the new Key before the old one expires.
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Not all user stations will have their time exactly synchronised, therefore
they
will keep the old Key until the delete time arrives. During this period a
station
will distinguish between the two different Keys by a Key serial number which
is included in the header. However once the delete time has arrived it will no
longer accept headers with an old Key.
Figure 1 shows a block diagram of a user station in the form of a radio
transceiver 10 with an associated Smart Card reader 12. The Smart Card
reader can be internal or external to the transceiver. The block diagram of
the
transceiver unit in Figure 1 corresponds essentially to the units described in
the abovementioned PCT patent application.
The unit includes a CPU 14 which is connected to an interface and modem
circuit 16. The unit includes multiple receiver modules 18 to 24 which can
receive incoming data at different data rates over a four decade range. The
unit includes an output/transmission module 26 which operates over the same
range, allowing the unit to operate at different data rates according to the
quality of the link between stations.
When the user station is turned on it must .first read the Smart Card to get
its
ID. It then checks if the Network Operator's Public Key has expired, or if the
ID in the smart card is different from the one it last used (it stores this
information on a local Flash Drive). If either of these two conditions is true
it
must then follow the procedure outlined in Figure 3 or 4. This entails
creating
a message which is then transmitted by the modems and transmitter to the
Network Operator station.
When the Smart Card is removed, the user station must stop operating. The
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Smart Card reader sends a message to the transceiver when the card is
removed. However, if the connection between the reader and the transceiver
is tampered with, the message would not arrive at the transceiver. To prevent
a transceiver from communicating without a Smart Card, the transceiver will
check the status of the Smart Card on a regular basis to make sure it has not
been removed. This will involve using the Smart Card to decode a random
number encoded with its Public Key. If the correct Smart Card is present it
will correctly decode the number. The random number will be encoded using
the software in the transceiver. Therefore if a user removes the Smart Card
after tampering with the line, the software in the transceiver will stop
running
after a predetermined interval.
Figure 3 shows the process through which the user station will get the
Network Operator's Public Key and its own Public and Private Keys. This
diagram assumes the use of a DES type of Smart Card. If an RSA type Smart
Card is used then the Network Operator would not generate Random A and
Result A, and would encrypt the Message with the RSA Public Key associated
with the user station ID. The user station will in turn decrypt the message
with
its Private Key. All the other steps remain the same. (See Figure 4 for a
diagram illustrating both options.) The RSA Smart Card will only be used to
get a new Public and Private Key. The new keys will be used when
communicating with other user stations and are subject to expire in the same
way as the Network Operator's Public Key.
When using a DES Smart Card the Result A is generated by applying the DES
algorithm to Random A. The Result A is then used to encrypt the whole
message. This allows the encryption of the whole message to be done using a
faster processor than the Smart Card. However if the Smart Card is used to
encrypt the whole message then Random A and Result A are not needed. In
T T
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this case the whole message is encrypted by the Network Operator using the
secret key associated with the user station. The user station will then
decrypt
the whole message using the Smart Card.
The Network Operator station is typically another transceiver unit which is
connected to a computer which keeps information on all the user stations
operating in the Network. This computer will generate the Public and Private
Keys for the user station, and it also keeps the Network Operator's Private
and
Public Keys. The Network Operator computer also contains all the numbers
associated with the Smart Cards in every user station. This allows the
Network Operator to send the Private Key of the user station back to the user
station without any other user station being able to retrieve the Key.
More than one Network Operator station may be present in a network and will
all be connected to the central Network Operator computer. One or more
backup Network Operator computers may also be connected to the Network
Operator station in case of failure.
When a user station is switched on for the very first time it will not have
the
current Network Public Key or its own Public and Private Key. Therefore it
needs to communicate with the Network Operator in order to get the Keys.
However, if it is not in the vicinity of the Network Operator it will not be
able
to send it a message as all other user stations will ignore the new user
station,
as they cannot verify it. Therefore a method is required to allow the other
user
stations to help the new one get its Keys without affecting their routing
tables
or jeopardising the security of the network.
When a user station is trying to get a new set of keys for the very first
time, it
will generate a special message for the Network Operator that must have a
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message number of one (1). This number will be reserved for the sole purpose
of retrieving Keys from the Network Operator.
When any other user station in the network sees this message it will create a
new ID in its routing tables which will be the same as the Origin ID of the
message, with an added flag indicating that it should not use this Flagged ID
for sending any data other than a response message from the Network
Operator which will also be numbered one (1). If this turns out to be a
"bogus"
or unauthorised user station, or if the user station in question was turned
off
long enough for the Keys to expire, but not long enough to be removed from
the routing tables, then two identical IDs will appear in the routing tables
of
the other user stations. The Flagged ID will be used for the routing of
messages with a number of one (1), and the other ID will be used for all other
messages.
The other user stations will also only allow these Key messages to pass if
they
are of the correct size, and have the correct Time-To-Die associated with
them. This will prevent an unauthorised user station from flooding a network
with many Key request messages. Since the message is small and will have a
long Time-To-Die associated with it, such an unauthorised station will only be
able to send out limited traffic.
If a user station is trying to get a Key update, but its current Keys are
still
valid, then it will request the new Keys without using a special message
number. This is required since the message requesting the Key will not be
small, as it will also contain billing information. Therefore the message will
be treated and routed like any other message.
In the same way that the message has a special number, so must the respective
1 I
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Probe and Data packets. This will allow the user station to introduce the
message into the network. However other user stations will only accept these
special probes because of the number of the Probe and Data Packet i.e. one
(1). Furthermore the other user stations will only accept a message of number
one (1) from such a Data packet. They will also add the same Flagged ID for
this type of Probe. The response Data packet from a legitimate user station
will also be numbered one (1). This will allow other user stations monitoring
the interaction to know they must Flag the ID associated with the response.
It is conceivable that an unauthorised Network Operator. station is set up
with
the purpose of infiltrating a network and intercepting messages. To prevent
this from happening a user station must be able to verify the authenticity of
the Network Operator. If a user station can not verify the Network Operator
then the user station will not allow itself access to the network.
In order for the user station to authenticate the new keys sent to it from the
Network Operator, the Network Operator must sign the new set of keys using
its Permanent Authority Private Key. The signed message can be verified by
the user station's Smart Card. Every such Smart Card has an Authority Public
Key. This key remains unchanged, and is permanently locked in the Smart
Card. A number of users may share the same set of Authority Keys. If the
Authority Keys should be recovered by a third party, that particular set can
be
removed from operation. This will mean that the users that share that
particular set of Authority Keys will have to get a new Smart Card in order to
continue using the network. It may be possible to assign each user their own
Authority Key set, thus reducing the number of users that would need to
update their Smart Card in the event of a security breach.
When a user station is switched on for the first time it has no billing
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information, and as such the Key Request Message will always be the same
size, hence other user stations will only accept one size for the Message.
However when the user station requests a new set of Keys because the current
ones are about to expire, it will also include billing information with the
request.
The billing information will include a list of user station IDs that the local
user
station has either sent data to or received data from. With each of the IDs
the
following details will be also sent:
* Total amount of data sent to remote transceiver ID.
* Total amount of data sent that was acknowledged by remote
transceiver ID.
* Total amount of data that was received from remote
transceiver ID.
* Special resources used (e.g. Internet Data)
* Statistics relating to power consumption, packet and message
errors, etc.
* Total amount of data sent on behalf of third party stations (ie.
relayed data).
This information will then be cross referenced by the Network Operator with
the billing information received from other transceiver ID's. This will then
be
used for determining how much to bill the user of each transceiver.
The network operator may credit a user who has actively relayed data on
behalf of other stations, thereby encouraging users to leave their stations
on.
The link level and/or message level of the above described protocol can be
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signed and/or encrypted. The Keys used in the method can be used for the
signing and/or encryption of the message header and/or the entire packet.
Each Data packet contains two CRCs. The first CRC is contained in the
header and is the CRC of the header. The second CRC is at the end of the
packet and is a CRC of the entire packet, including the header.
The reason for the use of two CRCs is to allow the protocol to determine the
origin of a packet if only the header comes through correctly, but the packet
is
invalid due to an error. Typically a station would first check the packet CRC.
If it were correct, it would then assume that the header CRC is also correct
(since the header is included in the packet CRC). If the packet CRC is in
error,
then the header CRC is checked. If the header CRC passes then the station can
assume that the information contained in the header is correct. This header
information could then be used for adaptive retransmissions even though the
data was lost.
In order to "sign" the packet, the CRC of the header and/or packet can be
encrypted using the Private Key of the sending station. The receiving station
would then decrypt the CRC using the Public Key of the sending station.
If the packet needs to be secured, then the whole header and/or packet can be
encrypted using the Public Key of the receiving station. The receiving station
would then decrypt the header and/or packet using its Private Key.
The header and/or packet could be both signed and secured by first encrypting
the CRC with the sending station's Private Key, then encrypting the entire
header and/or packet using the Public Key of the receiving station.
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The only part of the header and/or packet that would not be encrypted would
be the first part thereof, up to where the type of packet is identified, and
possibly up to the receiving ID (see the packet structure below). Thus a
station
would not try to decrypt every packet it receives, only those that are
indicated
as having been encrypted and/or signed (packet type). Furthermore, a station
which is not the receiving station would not try to decrypt the packet.
The protocol relies on the fact that a station can gather information from
third
party transmissions on a calling channel. Therefore the packet transmissions
would not be encrypted on the calling channel, only signed. However once
two stations move to a data channel, they can then both encrypt and sign the
packets.
Even if the packets are encrypted on the link layer, a third party at an
intermediate station is not prevented from analysing the packet after the
hardware has decrypted it. Therefore it would be important to encrypt any data
sent over the network at the message layer. In the case where the end user is
already using some form of encryption on their data it may not be necessary to
encrypt the messages.
When data enters the network (e.g. a user types a message at a terminal), the
message would be signed using the Private Key of the Origin Station, and
encrypted using the Public Key of the Destination Station. When the message
arrives at the Destination Station the message would be decrypted using the
Private Key of the Destination Station, and verified using the Public Key of
the Origin Station.
This signed and/or encrypted message would then remain unchanged as it
progresses through the network via intermediate stations. Therefore anyone at
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an intermediate station would not be able to access and/or tamper with the
contents of the message.
The Destination Station would typically only have the Public Keys of stations
in its immediate vicinity (from Key Probes). If the Destination Station does
not have the Public Key of the Origin Station it can send a Key Request
message to the Network Operator, requesting the Public Key of the Origin
Station.
The Network Operator would then send a message that contains the Origin
Station ID and its Public Key, encrypted using the Private Key of the Network
Operator. This would have the same effect as if the Destination Station heard
a
Key Probe from the Origin Station (see below).
For long packets and/or messages the RSA method of encryption would be
very slow. In these circumstances an alternative, faster encryption method
could be used, such as DES. The key used for the DES algorithm could then
be encrypted using the RSA Public Key of the Destination Station. The
encrypted key would then be attached to the message before signing it. The
Destination station would then extract the DES key using its RSA Private
Key. The extracted DES key would then be used to extract the entire packet.
Although a key length of between 16 and 128 bits is typical, it should be
noted
that longer keys could be used. However, longer keys require more
computation power, and also add additional overhead to the size of the packets
and messages. Therefore a compromise between key length, processing power
and packet size must be determined. Typically, as the power of computers
increases, so must the length of the keys be increased.
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It should be noted that the Private and Public Keys of both the user stations
and the Network Operator are changed at regular intervals, which means that
shorter keys can be used (the keys would probably already have changed by
the time someone "cracks" the code). Thus on the link level, shorter keys may
be used. However, for data security at the message layer longer keys may be
needed, assuming that it is required that the data must always remain secure,
even after transmission through the network.
In the above described system, the Network Operator Key expires. When a
station gets the new Network Operator Key, the station itself also receives a
new key. Therefore the key serial number of the Network Operator would also
apply to the station's keys. However it would be possible to assign a separate
key serial number for the user stations, thereby allowing the user station's
keys
to remain valid for longer or shorter times, as required. The user station
would
still follow the same procedure to acquire a new key, but the Network
Operator Serial number would remain the same, and the user serial number
would change (or vice versa).
In the above described system, the CRC of a message or packet is signed.
However, a more secure method would be to use a hash function that would
create a message digest or digital fingerprint of the data to be signed. The
advantage of the hash function is to make it harder to create an altered
message that generates the same hashed value. It will be appreciated that
either a CRC function or a hash function could be used for the signing of
packets or messages.
Below is the basic structure of the probe and data packets used in the method
of the invention:
I I 1
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Probe or Data Packet
Variable Bit Len Allows
Preamble 64 Modem training sequence (101010101010 etc ...)
Syncl 8 1st Sync character for packet detection
Sync2 8 2nd Sync
Sync3 8 3rd Sync
Packet Size 16 Size of packet
Size Check 8 Packet Size Check
Protocol Version 8 Protocol Version 1->255
Packet Type 8 Indicates Type and if header and/or packet signed and/or
encrypted
Sending ID 32 ID transmitting packet
Receiving ID 32 ID to receive packet
Packet Number 16 Packet number, 1->65 535
Adp Parameters 72 Adaptation Parameters used by Parrot Link Layer
Header CRC 16 16 to 128 bits, depending on level of security required
Data x Contains higher level data for protocol
CRC 32 32 to 128 bits, depending on level of security required
Probe packets (containing no data) would typically be sent out on a calling
channel requesting a response from Destination Station having a particular ID
(Receiving ID). The probe packets would typically not be encrypted, but
would be signed, thus allowing other stations to gather required information
for routing.
When a station responds to a probe, it would do so on a data channel using a
Data packet (containing data). The Data packet would be signed, and could
optionally be encrypted, since no other station needs the information
contained therein.
The length of the packet CRC is set at a minimum of 32 bits for reliable error
detection at the link level.
SUBSTITUTE SHEET (RULE 26)
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Key Probe Packet
Variable Bit Len Allows
Preamble 64 Modem training sequence (101010101010 etc ...)
Sync1 8 1st Sync
Sync2 8 2nd Sync
Sync3 8 3rd Sync
Packet Size 16 Size of packet
Size Check 8 Packet Size Check
Protocol Version 8 Protocol Version 1->255
Packet Type 8 Indicates Type (Key Probe)
Sending ID 32 ID of station transmitting packet
Network Key Serial 8 Network Operator Public Key Serial Number
Encrypted ID & Key x Sending ID & Public Key Encrypted (x = 56 to 168 bits)
CRC 32 32 bit CRC for whole packet, including header
Key Probes are sent out to indicate the Public Key of a station. They are sent
out at regular intervals on a probing channel in place of normal probes. Other
stations will use the Key Probes to determine the Public Key of other
stations.
The ID (32 bits), User Level (8 bits), and Public Key (16 to 128 bits) of the
Sending Station are encrypted using the Network Operators Private Key. Thus
other stations can verify the Public Key of the Sending Station by decrypting
the message with the Public Key of the Network Operator.
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Format of Key response message from Network Operator
Message Data Bit Len Description
Message Type 8 Message Type = Key Response
Data 1 User ID 32 ID of station requesting key
Data 1 User Level 8
Data 1 User Public Key x Public Key (x = 16 to 128 bits)
Data 2 User Private Key x Private Key (x = 16 to 128 bits)
Data 2 Network Serial No 8 Network Key Serial Number
Data 2 Network Renewal 16 Renewal Time in seconds (18 hours max)
Data 2 Network Expire 16 Expire Time in seconds (18 hours max)
Data 2 Network Delete 16 Delete Time in seconds (18 hours max)
Data 2 Network Public Key x Private Key (x = 16 to 128 bits)
Message Checksum 16,
The Key response message is sent from the Network Operator to the user
station requesting a key update. The data items marked Data 1 in the above
table are encrypted using the Network Private Key. This means that any
station with a valid Network Public Key would be able to extract the ID,
Public Key, and User Level of the station transmitting this information in the
Key Probe Packet.
Data 2 and the encrypted Data 1 are combined and encrypted with the Public
Key of the requesting station's RSA Smart Card (or Result A when using a
DES Smart Card - see Figure 4). The requesting station would then be able to
extract the contents using its Private Key from the Smart Card. This is the
only time that the Smart Card keys are used. The keys included in the Key
response message will be used for all other signing and encryption. The length
of the Smart Card key would typically be very long (e.g. 1024 bits) since this
key will never change (unless the Smart Card is changed).
The renewal, expire, and delete times are all measured in relative seconds.
When a user station requests a key update, the Network Operator calculates
SUBSTITUTE SHEET (RULE 261
CA 02280906 1999-08-09
WO 98/35474 PCT/GB98/00392
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the relative time left until the current key must be renewed, etc. It places
these
respective relative time in seconds into the message. When the user station
receives the message it subtracts the time the message spent in the network
from the respective times. It then determines the absolute time at which the
key must be renewed, expired, and deleted relative to its local clock.
The reason for using relative time is that less bits are required to indicate
the
time, and secondly not all user stations would have their clocks correctly
synchronised. The time a message spends in the network can be accurately
determined by the network protocol (typically within a few milliseconds).
The use of absolute times would work equally well, provided the clocks at the
user stations can be kept reasonably synchronised. However, the
authentication method described in this document allows for overlap of clocks
that are not accurately synchronised.
i
